1. Field of the Invention
The present invention relates to a display for a computer monitor, and a liquid crystal display device for displaying video images etc., and more particularly to a liquid crystal display device having an excellent viewing angle characteristic.
2. Description of the Related Art
There are two known techniques for improving or widening the viewing angle of a liquid crystal display device: (1) changing orientation directions of liquid crystal molecules within a plane substantially parallel to a substrate surface; and (2) in a display mode where orientation directions of liquid crystal molecules are changed to a direction perpendicular to a substrate surface, dividing into domains having different orientation directions (i.e., azimuthal angle direction) of liquid crystal molecules in a plane parallel to the substrate surface. A typical example of the former technique is the In-Plane-Switching (IPS) mode. Examples of the latter technique include: (1) a wide viewing angle liquid crystal display mode in which each pixel has a liquid crystal region where Np-type liquid crystal (nematic liquid crystal having positive dielectric anisotropy), which is oriented horizontally with respect to a substrate surface in the absence of applied voltage, is oriented axially symmetrically (Japanese Laid-Open Publication No. 7-120728); (2) a wide viewing angle liquid crystal display mode in which Nn-type liquid crystal (nematic liquid crystal having negative dielectric anisotropy), which is oriented substantially perpendicular to a substrate surface in the absence of applied voltage, is divided into a plurality of domains in which the falling directions in the presence of applied voltage are different (Japanese Laid-Open Publication No. 7-64089); and (3) a wide viewing angle liquid crystal display mode in which Np-type liquid crystal molecules in each pixel are divided substantially equally into four and are horizontally oriented (AM-LCD ""96, p. 185(1996)). In the latter technique, a phase difference compensation element is requited on a principle so as to compensate a viewing angle of a 45xc2x0 direction with respect to the absorption axis of a polarizer element.
Herein, the term xe2x80x9cphase difference compensation elementxe2x80x9d means an optical component having birefringence in the form of a plate, sheet, or film. The term xe2x80x9cpolarizer elementxe2x80x9d means an optical component which absorbs one of two orthogonal beams of polarized light and passes the other beam. The absorption axis and transmission (polarization) axis of the polarizer are orthogonally crossed.
A phase difference film is currently made of stretched film. As resin material of the stretched film, polyvinylalcohol (PVA) and polycarbonate (PC) are commonly used. The phase difference film is required to have the following optical and mechanical characteristics,
(1) optical characteristics: less nonuniformity of phase difference, ability to match a wavelength dispersion characteristic of birefringence of a liquid crystal layer, ahigh level of heat resistance and moisture resistance, no optical defects such as light axis disturbance and contaminants, ahigh level of transmission, a small photoelasticity coefficient, performance of resisting transmission deterioration due to ultraviolet light, and the like; and
(2) mechanical characteristics: a high level of elasticity, a high level of tensile strength, a high level of yield bending strength, and the like.
The phase difference film is also required to be easy to process (e.g., easy to stretch) in the course of its manufacture. Among the above-described required characteristics, practically important characteristics are less phase difference nonuniformity, ability to match a wavelength dispersion characteristic of birefringence of a liquid crystal layer, and a small photoelasticity coefficient.
Japanese Laid-Open Publication No. 6-3524 discloses a uniaxial optical phase difference film made of crystalline copolymer of monochlorotrif luoroethylene (80-98 wt %) and fluorovinylidene (2-20 wt %) which is aligned in one direction.
The inventors have found that liquid crystal display devices using the above-described conventional phase retardation member have the following problems.
A phase retardation member disclosed in Japanese Laid-Open Publication No. 6-3524 cannot satisfy a retardation condition necessary for improving the display quality of a liquid crystal display device with a large screen (e.g., 42 inch type) or a liquid crystal display device including a liquid crystal layer having two or more different kinds of liquid crystal regions taking different initial orientation states in each of a plurality of pixels; or a liquid crystal layer having a liquid crystal region in which orientation directions of liquid crystal molecules continuously vary. In other words, when the phase retardation member is applied to these liquid crystal display devices, an insufficient viewing angle compensation effect is obtained and thus a viewing angle is so narrow that an image on the display screen becomes yellowish when viewing from a slanted direction. It is also difficult to manufacture the phase retardation member without nonuniformity of color having such a size that the member may be applied to a liquid crystal display device with a large screen (e.g., 42 inch type). Herein below, problems that the inventors have found will be described in detail with reference to the accompanying drawings.
In a large-size display device, e.g., 42-inch liquid crystal display device, various stresses are put on a phase difference compensation element. The various stresses are, for example, (1) a stress generated by attachment of a polarizer to the phase difference compensation element, or attachment of a liquid crystal cell to a polarizer attached to the phase difference compensation element; and (2) a stress due to heat generated by backlighting. As a result, the phase difference compensation element has retardation caused by the locally occurring stresses. This state is fixed by an adhesive. As a result, the transmission of the device is locally increased in a crossed-Nicols arrangement of the polarizers, whereby the brightness of the device becomes irregular and thus its display quality is largely impaired.
FIG. 1 schematically illustrates a display surface 10 of a large-size liquid crystal display device. FIG. 1 shows local areas having increased transmission (light leakage) caused by the above-described stresses in a black display state. Light leakage (type 1) caused by a stress generated by attachment of a polarizer to a phase difference compensation element often emerges around the center of each of four edges of the display surface 10. Light leakage (type 2) caused by a stress due to heat generated by backlighting emerges at the four corners of the display surface 10. The degree of light leakage caused by these stresses depends on the magnitudes of the stresses and the photoelasticity coefficient of the material of the phase difference compensation element.
In the above-described display mode in which a pixel is divided into domains having different orientation directions of liquid crystal molecules, there is a problem that a viewing angle characteristic in a direction of an axis bisecting an angle between the absorption axes of an upper polarizer and a lower polarizer both sandwiching a liquid crystal cell (a viewing angle of a 45xc2x0 direction with respect to the absorption axis of a polarizer) becomes significantly worse compared with a viewing angle characteristic in a direction of the absorption axis (see FIG. 14 that illustrates isocontrast contour curves of Comparative Example 1 which will be described below).
Positions of polarizers in a liquid crystal display device and a definition of a direction of a viewing angle will be described with reference to FIGS. 2A and 2B.
FIG. 2A is a diagram schematically illustrating a crossed-Nicols arrangement of polarizers in a liquid crystal display device. A polarizer disposed at a side nearer a viewer of a liquid crystal cell 20 is referred to as an upper polarizer. A polarizer disposed at a side nearer a backlight of a liquid crystal cell 20 is referred to as a lower polarizer. The absorption axis 22a of the upper polarizer is orthogonal to the absorption axis 22b of the lower polarizer. As shown in FIG. 2B, a viewing direction (a direction of a viewer""s line of sight 20) is defined by (1) an angle from a normal line 26 with respect to a virtual plane 24 parallel to the display surface of a liquid crystal display device (viewing angle xcex8) and (2) an angle from a direction 28 of the absorption axis of the upper polarizer (azimuthal angle "PHgr": the anticlockwise direction is positive). Contrast ratios are evaluated with respect to xcex8-"PHgr" to obtain isocontrast contour curves irrespective of the display mode. Note that an x-y-z orthogonal coordinate system is used to define the anisotropy of an optical characteristic of a liquid crystal display device where the z axis is along a direction of the normal line 26 of a virtual plane 24; the x axis is along a direction of "PHgr"=270xc2x0 in the virtual plane 24; and the y axis is along a direction of "PHgr"=0xc2x0 in the virtual plane 24.
As shown in FIG. 3A, a commercially available polarizer 30 generally includes a polarization layer 32 sandwiched between supporting films 34 and 36. The polarization layer 32 (e.g., made of PVA) has less strength and is therefore supported by the supporting films 34 and 36 (e.g., made of triacetylcellulose (TAC)). As shown in FIG. 3B, a direction 32a of the absorption axis of the polarization layer 32 is almost the same as directions 34a and 36a of the slow axes of the respective supporting films 34 and 36.
A liquid crystal display device according to the present invention includes a liquid crystal cell having a pair of substrates and a liquid crystal layer, the liquid crystal layer being sandwiched between the pair of substrates; a pair of polarizer elements sandwiching the liquid crystal cell; and a phase difference compensation element provided between the liquid crystal cell and at least one of the pair of polarizer elements. A photoelasticity coefficient of the phase difference compensation element is 10xc3x9710xe2x88x9213 cm2/dyne or less.
In one embodiment of the present invention, the phase difference compensation element has three refractive indices nx, ny, and nz in directions of x, y, and z axes which are orthogonal to each other, and the refractive indices nx, ny, and nz satisfies a relationship nz less than ny less than nx where the nx and ny are principal refractive indices in a plane parallel to a surface of the liquid crystal cell and the nz is a principal refractive index in the thickness direction of the liquid crystal cell.
In one embodiment of the present invention, the liquid crystal cell has a plurality of pixels, and each of the plurality of pixels has two or more liquid crystal domains having different initial orientation states or has a liquid crystal region where orientation directions of liquid crystal molecules therein continuously vary. The liquid crystal display device includes two phase difference compensation elements, a first phase difference compensation element being sandwiched between the liquid crystal cell and one of the pair of polarizer elements and a second phase difference compensation element being sandwiched between the liquid crystal cell and the other of the pair of polarizer elements. Each of the first and second phase difference compensation elements include a phase difference film. The phase difference film has the slow axis in a plane parallel to a surface of the liquid crystal cell. The slow axes in the plane parallel to a surface of the liquid crystal cell, of the first and second phase difference compensation elements are orthogonally crossed.
In one embodiment of the present invention, the liquid crystal display device includes two phase difference compensation elements, a first phase difference compensation element being sandwiched between the liquid crystal cell and one of the pair of polarizer elements and a second phase difference compensation element being sandwiched between the liquid crystal cell and the other of the pair of polarizer elements. Each of the first and second phase difference compensation elements has the slow axis in a plane parallel to a surface of the liquid crystal cell. The slow axis of the first phase difference compensation element is orthogonal to the absorption axis of one of the pair of polarizer elements provided on the same side with respect to the liquid crystal cell as the first phase difference compensation element, and the slow axis of the second phase difference compensation element is orthogonal to the absorption axis of one of the pair of polarizer elements provided on the same side with respect to the liquid crystal cell as the second phase difference compensation element.
In one embodiment of the present invention, the liquid crystal layer includes nematic liquid crystal material. Liquid crystal molecules of the nematic liquid crystal material are, in a black display, oriented in a direction substantially perpendicular to a surface of the pair of substrate.
In one embodiment of the present invention, the liquid crystal layer includes nematic liquid crystal material having negative dielectric anisotropy. Liquid crystal molecules of the nematic liquid crystal material are, in the absence of applied voltage, oriented in a direction substantially perpendicular to a surface of the pair of substrates.
In one embodiment of the present invention, the liquid crystal display device satisfies relationships
0.11 less than {dfxc2x7(nxxe2x88x92nz)}/(dLCxc2x7xcex94n) less than 0.75 and
xe2x80x830 less than {dfxc2x7(nxxe2x88x92ny))/(dLcxc2x7xcex94n) less than 0.26
where xcex94n is a birefringence of the liquid crystal molecule of the liquid crystal layer, dLC is an average thickness of the liquid crystal layer, and df is a thickness of the phase difference compensation element.
In one embodiment of the present invention, each of the pair of polarizer elements is a polarizer layer formed on a supporting film, and the supporting film serves as the phase difference compensation element.
In one embodiment of the present invention, the phase difference compensation element includes liquid crystal polymer.
In one embodiment of the present invention, the liquid crystal cell has a plurality of pixels, and each of the plurality of pixels has two or more liquid crystal domains having differen t initial orientation states or has a liquid crystal region where orientation directions of liquid crystal molecules therein continuously vary. The liquid crystal display device includes two phase difference compensation elements, a first phase difference compensation element being sandwiched between the liquid crystal cell and one of the pair of polarizer elements and a second phase difference compensation element being sandwiched between the liquid crystal cell and the other of the pair of polarizer elements. Each of the first and second phase difference compensation elements include a plurality of layered phase difference films, each of the plurality of phase difference films having the slow axis in a plane parallel to a surface of the liquid crystal cell. The slow axes in the plane parallel to a surface of the liquid crystal cell, of the first and second phase difference compensation elements are orthogonally crossed to each other.
In one embodiment of the present invention, the phase difference film has a layered structure in which two or more uniaxial films are attached to each other. At least one of the uniaxial films has retardation in the plane and in a direction of a normal line.
In one embodiment of the present invention, the phase difference film has a structure in which material having refractive index anisotropy is layered on a supporting film.
Thus, the invention described herein makes possible the advantages of providing a liquid crystal display device having substantially no nonuniformity of brightness due to the stresses wherein the device has a structure in which a phase difference compensation element is attached to a polarizer or to a liquid crystal panel, and therefore having uniform display quality.
Herein below, the action of the present invention will be described.
Providing a phase difference compensation element having a photoelasticity coefficient of 10xc3x9710xe2x88x9213 cm2/dyne or less can reduce generation of nonuniformity of polarization in a device having a structure in which a phase difference compensation element is attached to a polarizer, or a combination of the polarizer and the phase difference compensation element is attached to a surface of a liquid crystal cell, thereby obtaining uniform display quality. The phase difference compensation element of the present invention has non-zero birefringence, and preferably an average refractive index in the range of 1.4-1.7.
When the principal refractive indices nx, ny, and nz of the phase difference compensation element satisfies a relationship nz less than ny less than nx, a viewing angle characteristic in a 45xc2x0 direction with respect to the absorption axis of a polarizer can be improved.
To reduce dependence of a viewing angle characteristic on azimuthal angles, it is effective that the refractive index anisotropy in the short axis direction of a liquid crystal molecule is decreased. To this end, two or more domains having different orientation states of liquid crystal may be provided in each pixel. Further, when the orientation states of liquid crystal molecules in each pixel vary continuously (axially symmetrical or radial orientation), the refractive index anisotropy in the short axis direction of a liquid crystal molecule is decreased to a minimum. In this case, the dependence of a viewing angle characteristic on azimuthal angles can be improved to a maximum. It is desirable that the phase difference compensation element consists of one sheet of phase difference film from the view point of cost and fabrication process, although the phase difference compensation element may include a plurality of sheets of phase difference film.
In the case where one phase difference compensation element is provided in each of the spaces between the liquid crystal cell and a pair of the polarizers, when the slow axes of a pair of the phase difference compensation elements are orthogonally crossed, in-plane phase difference does not occur between the phase difference compensation elements and the liquid crystal cell, thereby preventing reduction of the contrast ratio. When the absorption axis of the polarizer and the slow axis of the phase difference compensation element are orthogonally crossed, the viewing angle compensation effect in a 45xc2x0 direction with respect to the absorption axis of the polarizer is further effectively obtained.
When liquid crystal molecules are oriented in a direction perpendicular to the substrate surface, the refractive index anisotropy in a plane of the liquid crystal cell is small. Therefore, light leakage in a black level is reduced even when a viewing angle is fallen in a 45xc2x0 direction with respect to the absorption axis of the polarizer, thereby preventing from lowering the contrast and achieving further compensation of a viewing angle. This effect is significant in a normally black mode display device which employs an Nn liquid crystal material having negative dielectric anisotropy.
Specifically, for example, when 0.11 less than {dfxc2x7(nxxe2x88x92nz)}/(dLCxc2x7xcex94n) less than 0.75 and 0 less than dfxc2x7(nxxe2x88x92ny)}/(dLCxc2x7xcex94n) less than 0.26 are satisfied, where xcex94n is the birefringence of liquid crystal molecules of a liquid crystal layer, dLC is the average thickness of the liquid crystal layer, and df is the thickness of a phase difference compensation element, a significant effect is obtained.
The supporting film of the polarizer has retardation in a plane of the film and in the normal direction of the film. When the retardation of an additional phase difference compensation element is designed by taking into account the retardation of the supporting film, an optimal retardation can be obtained where the viewing angle compensation effect is maximized. The phase difference compensation element can be used as the supporting film of the polarizer. In this case, the phase difference compensation element is made in the fabrication process of the polarizer. This is advantageous from the viewpoint of cost and the process of attaching to a liquid crystal panel.
The liquid crystal polymer is an optically anisotropic molecule. The molecule is oriented on an orientation layer of a substrate. Therefore, a desired retardation can be easily obtained by coating a substrate (e.g., supporting substrate) with such liquid crystal polymer without stretching the film. This may lead to a reduction in cost. The refractive index anisotropy of a film can be variously set by controlling the orientation of the liquid crystal polymer.